This invention relates to image correction of a cathode-ray tube display, and particularly to a digital circuit for dynamic focus and astigmatism correction.
The focusing of an electron beam by electric or magnetic fields in a cathode-ray tube, hereinafter abbreviated CRT, is closely analogous to the focusing of light rays in an optical system. Not surprisingly, the electro-optical system of a CRT is plagued by distortions very similar to those which plague optical systems.
Two of the more troublesome distortions are defocusing and astigmatism. Astigmatism is a focal defect in which the electrons in different axial planes come to focus at different points. Under these conditions the CRT spot is not round, thus causing different trace widths depending upon the direction of the trace. More information concerning the various CRT distortions may be found in the book entitled The Cathode-Ray Tube by G. Parr and O. H. Davie, Reinhold, 1959.
A number of schemes have been developed to dynamically correct for image distortions in a CRT. These schemes generally fall into one of two categories, either analog correction systems or digital correction systems. One type of system which is representative generally of the analog approach has been developed by the present inventor and is discussed in U.S. Pat. No. 4,249,112, entitled "Dynamic Focus and Astigmatism Correction Circuit" issued Feb. 3, 1981, assigned to Tektronix, Inc. In that system analog circuitry is relied upon to generate image correction waveforms for dynamic focus and for astigmatism, these waveforms being applied to the CRT to effect the desired corrections. Although in general the waveforms required to correct the image at all points on the CRT screen can be relatively complicated mathematical functions, it is customary to approximate these functions by low order polynomials having either constant or regionally constant coefficients. Hence, the accuracy of the corrections depends on how well these polynomials fit the more complicated mathematical functions which represent the image correction function. The result is that images in some regions of the CRT screen are corrected much better than images in other regions of the screen. Also, this approach as well as analog approaches used by others, does not attempt to obtain optimum focus at a large number of independent points which are relatively uniformly distributed on the screen, an approach which would require much more complicated waveforms than can be represented by a small number of low order polynomials.
Such an approach, however, is the typical one used in digital correction schemes. For example, in U.S. Pat. No. 4,099,092, issued July 4, 1978, entitled "Television Display Alignment System and Method", by Stephen D. Bristow, assigned to Atari, Inc., a preprogrammed set of correction signals is used to correct aberrations at a large number of points on the screen. First, an alignment signal is applied to the CRT deflection plates instead of the normal scan signal, and the position of the beam on the screen is monitored. Then, when the beam is at preselected points, correction signals are computed and stored in digital form in a programmable read only memory or ROM. During normal operation these correction signals are converted to analog form and applied to the CRT to correct aberrations in accordance with scan signals which correspond to the position of the beam on the screen.
A similar approach described in U.S. Pat. No. 3,740,608, issued June 19, 1973 entitled "Scanning Correction Methods and Systems Utilizing Stored Digital Correction Values", by Manber et al., assigned to Alphanumeric Incorporated. There, digital correction values are stored corresponding to the corrections required in particular regions of the CRT screen. As the beam is commanded to a new position on the screen, the appropriate correction value for that region is called from a memory and is converted to an analog correction signal by a digital to analog converter.
Another digital approach is represented by U.S. Pat. No. 3,648,077, issued Mar. 7, 1972, entitled "Digital Cathode-Ray Tube Linearity Corrector" by Jerry Dale Merryman, assigned to Texas Instruments Incorporated. in that approach, the system utilizes digital circuitry to produce a correction factor from X- and Y-coordinate data supplied by a digital computer. The correction factor, which is equivalent to the sum of the squares of the X- and Y-coordinate data, is converted into an analog signal and is multiplied by and added to the analog coordinate signals. This produces the corrected deflection signals to eliminate pin-cushion distortion of images displayed on a CRT screen having an essentially flat surface. A serious drawback to this approach, however, is that it requires knowledge of the analytical form of the correction signal in order to calculate it with the computer. For complicated aberrations such an analytical form is typically unknown, so that the method of Merryman is not generally applicable.
Another approach which combines some of the features of the analog approach and the digital approach is described in U.S. Pat. No. 4,354,143, issued Oct. 13, 1982, entitled "Equipment to Correct Aberrations of a Cathode Ray Beam" by Ian D. Judd, assigned to International Business Machines Corporation. That reference discloses an apparatus by which aberrations are corrected as the beam is scanned across the screen of a CRT by deriving correction signals from stored digital values using the calculus of finite differences. Digital values, which are the initial differences of polynomial correction functions, are stored in memory. Then as the electron beam scans the screen horizontally new values of the correction function are calculated and applied to the CRT for each zone into which the CRT screen has been divided. During line flyback, changes in the correction function due to changes in Y-portion are calculated. Although this approach is more generally applicable than that of Merryman, it still assumes low order polynomials are satisfactory for the correction functions. In addition, the particular scheme for addressing the stored digital values is not disclosed.
Yet another reference which describes a digital correction scheme is U.S. Pat. No. 4,388,619 issued June 14, 1983, entitled "Corrector for Bundle Deflection Distortion in Multibeam Cathode Ray Tubes", by Vernon D. Beck, assigned to International Business Machines Corporation. Although a major concern of that patent is in the use of a split focus coil for correcting distortion due to undesired rotation of an array of electron beams, it discloses a digital correction scheme for applying correction signals to the focus coil which is similar to those already described. Correctional currents are supplied to the split focus coil as a function of the matrix beam displacement on the CRT screen. Correction signal values are stored in a memory, the values corresponding to the two correction currents to be supplied to each half of the split focus coil. An address translation means is then provided which synchronizes the addressing of the memory with the X- and Y-deflection signals to the deflection yoke so that the appropriate portion of the memory is accessed relative to the position of the scan on the CRT screen. There is no disclosure, however, as to the design of such an address translation means.
In each of the above references pertaining to digital correction systems, a significant requirement is that each particular correction signal applied to the CRT correspond to a particular location of the electron beam on the CRT screen. Nevertheless, little effort appears to have been devoted to obtaining a simple addressing apparatus for achieving that correspondence between the correction signals and the beam location.